PIV (Particle Image Velocimetry), in which the flow field of a fluid containing tracer particles is irradiated with a two-dimensional laser sheet at a time t1 and at a time t1+Δt, an image of the tracer particles within the laser sheet at each of these times is taken by a camera, and the velocity of the tracer particles within the laser sheet is determined based on the difference in luminance pattern of the tracer particles between the images at these two times, is known.
In such a particle image velocimetry system, it is desirable to take an image of only light reflected from the tracer particles contained in the fluid, but in reality since it is inevitable that images of noise components such as halation, light reflected from an object placed in the fluid, and light reflected from dirt attached to an observation window of a wind tunnel are taken together with that of light reflected from the tracer particles, there is a possibility that, when the velocity vector of the fluid is calculated from the camera images, an erroneous velocity vector (erroneous vector) will be calculated.
An arrangement in which, in order to minimize the above calculation of an erroneous vector, an image of only light reflected from the tracer particles is obtained by removing as noise from an original image the time-average luminance value of each pixel of the image or the minimum luminance value among a plurality of images taken at different times, is known from Patent Document 1 below.
Furthermore, an arrangement in which an image of only tracer particles is left by filtering spatial frequency components from gray-scale images at two consecutive times of the camera by means of a high pass filter and a low pass filter, thus obtaining the maximum travel distance of the gray-scale images at the two times with good precision, is known from Patent Document 2 below.
Moreover, as a particle image velocimetry system, an arrangement in which a space having floating tracer particles is irradiated with two two-dimensional laser sheets that are parallel to each other and have different wavelengths, images of tracer particles present within the laser sheets are taken by two cameras placed in directions perpendicular to the laser sheets, and by comparing the luminance patterns of the tracer particles of the two images obtained, three velocity components containing velocity components in the X direction and the Y direction within the plane of the laser sheet (in-plane velocity components) and a velocity component in the Z direction perpendicular to the laser sheet (out-of-plane velocity component) are determined is known from Patent Document 3 below.
Similarly, as a particle image velocimetry system, a so-called stereo PIV in which a three dimensional velocity field is irradiated with one laser sheet, images of tracer particles present within the laser sheet are taken at two times by two cameras from two different directions with respect to the plane of the laser sheet, and three velocity components, that is, in-plane velocity components and an out-of-plane velocity component, are obtained by comparing luminance patterns of the tracer particles of the two images obtained is known from Patent Document 4 below.
Furthermore, when an image of tracer particles irradiated with the laser sheet is taken by imaging means, if an image of reflected light that has been reflected from an object as a background therefor is taken by the imaging means together with reflected light that has been reflected from the tracer particles, the reflected light from the object becomes noise to thus decrease the S/N ratio of the reflected light, thus causing the problem that the measurement precision for the flow velocity is degraded.
In order to solve this problem, an arrangement in which fluorescence generated by employing laser light as excitation light is generated by impregnating tracer particles, which are solid particles of silica (SiO2), with a fluorescent material, and an image of the tracer particles is taken via a filter that only passes the wavelength of this fluorescence to thus block reflected light that has been reflected from an object as a background, thereby obtaining a clear image of the tracer particles, is known from Patent Document 5 below.
Furthermore, an arrangement in which a Laskin nozzle is used as a tracer particle generating device of a particle image velocimetry system is known from Non-Patent Document 1 below. In this tracer particle generating device, compressed air is supplied via a Laskin nozzle disposed in an oil stored in a pressure vessel, thus generating air bubbles containing oil droplets, and the air bubbles burst on the liquid surface of the oil to thus make the generated oil droplets collide with a collision plate, thus making the particle size uniform, after which they are supplied as tracer particles.    Patent Document 1: Japanese Patent Application Laid-open No. 2001-74415    Patent Document 2: Japanese Patent Application Laid-open No. 2008-140103    Patent Document 3: Japanese Patent Application Laid-open No. 10-19919    Patent Document 4: Japanese Patent Application Laid-open No. 2004-286733    Patent Document 5: Japanese Patent Application Laid-open No. 5-297014    Non-Patent Document 1: PIV Handbook, Ed. by The Visualization Society of Japan, Morikita Publishing Co., Ltd.